.sup.222 Rn is a radioactive decay product of .sup.238 U which occurs naturally in the earth's crust and especially in granite rocks. .sup.222 Rn is often referred to simply as "radon" and that terminology will hereinafter be employed. That is, radon, as herein used, is defined to mean the specific isotope .sup.222 Rn which has a half-life of 3.82 days, decaying predominantly to the isotope .sup.218 Po with the emission of an alpha particle of 5.49 MeV of energy.
Radon is the heaviest of the inert gases, the end of the series beginning with helium and neon. When produced, it has the properties and the lifetime to diffuse out the minerals in which it forms and becomes a constituent of the air we breathe. Techniques for its collection and measurement date from its discovery in 1902. Private and public actions to understand and alleviate, ameliorate, or mitigate the problem require accurate measurements of the radon concentration in buildings. The effectiveness of charcoal to adsorb radon has been known since around 1910. However, only one paper: H. M. Prichard and K. D. Marien, "Desorption of radon from activated Carbon into a Liquid Scintillator," Analytical Chemistry 55, 155-157, 1983, describes the use of liquid scintillation counting techniques to measure the radon adsorbed in the charcoal. Liquid scintillation counting of radon makes use of the fact that the radon can be eluted from the charcoal into a solvent such as toluene or zylene since these chemicals have a far greater affinity for radon than does charcoal. A scintillation liquid such as Econofluor, available from Dupont DeNemours and Co., containing PPO-POPOP scintillants, for example, can be added to the elutant such as toluene so that each ionization event in the counting liquid results in a pulse of light which can be detected in a photomultiplier photon counter. The pulses can then be analyzed by techniques well known in the art.
Prichard and Marien recognize that moisture uptake by the charcoal can be a problem but do not consider ways to solve the problems of moisture on the measurements of the radon. Hereinafter, there will be described practical devices for effectively using the liquid scintillation technique for measuring radon concentration in activated charcoal exposed in domestic and commercial buildings. Particularly there will be described devices which reduce and eliminate the moisture uptake problems which seriously compromise the use of activated charcoal for radon adsorption.
The complex chain of radioactive decay events which follows the decay of radon explains why the liquid scintillation technique is inherently 2.5 times more effective than the almost universally used gamma ray techniques for measuring the radon in charcoal. Examination of the practicalities of gamma ray measurements of radon in charcoal shows that liquid scintillation counting of the alpha and beta particles has additional advantages which make liquid scintillation at least 25 and generally almost 100 times more effective than gamma ray measurements.
Radon, being inert, is not itself considered a health hazard. The harmful effects result mainly from the decay radiations from the progeny of the radon, all of which are chemically very active. There are five sequential decays which occur in the first few hours following the decay of .sup.222 Ra. The immediate daughter of .sup.222 Ra is .sup.218 Po which tranmutes in 3.05 minutes into .sup.214 Pb by emitting a 6.0 MeV alpha particle. .sup.214 Pb decays in turn with a half-life of 26.8 minutes, with the emission of an electron and a gamma ray, to an isotope of bismuth, .sup.214 Bi, which itself decays to .sup.214 Po in 19.8 minutes by emitting an electron and a gamma ray. Finally, .sup.214 Po decays in 164 microsecs by emitting a 7.687 MeV alpha particle. In summary, a sequence of short-lived transmutations takes place following each radon decay, and each step yields easily detectable radiation. In a matter of hours, three alpha particles, two electrons, and about two gamma rays are emitted for every radon decay. Thus, liquid scintillation (measuring both alpha and beta particles) can detect at least 2.5 times as many events as can gamma counters.
The dangers posed by the radiations emitted when radon decays have prompted the United States Environmental Protection Agency to issue guidelines for the levels of radon in air permissible under various circumstances. In domestic environments, an average yearly radon concentration exceeding 4 picoCuries per liter of air (4 pC/l) is considered cause for concern. A picoCurie is 3.7.times.10.sup.-2 disintegrations per second, or 3.7.times.10.sup.-2 Bq, where Bq is the symbol for a Bequeral, defined as one disintegration per second.
It is convenient to divide the methods and devices which have been developed for the detection of low levels of radon into passive and active. The passive methods, such as the invention to be described herein, make use of passive gas diffusion into any one of a variety of devices, so as to accumulate the radon or accumulate the effects of the radon emanations, for later measurement or analysis. Active methods have an active component at the test site. The component may be the gatherer device for the radon, it may be the electronic detector of the radon, it may, and generally does have both an active gatherer and an active detector. These methods are not relevant to this patent.
The passive methods and devices accumulate the radon, or effects resulting from the radon emanations, by passive diffusion of the ambient radon-bearing air into the accumulator. Measurements are made in a laboratory environment. For example, plastic track detectors accumulate track evidence for the number of alpha particles emitted in the decay of radon over a period of time. Plastic track detectors are still marketed but represent a fraction of the market. The overwhelming share of the passive radon detection market uses charcoal to accumulate the radon, and gamma ray detection techniques to measure the radon concentration.
The gamma rays emitted in the third and fourth links of the radon decay chain, i.e., from the decays of .sup.214 Pb and .sup.214 Bi, give a unique measure of the radon content. In this prior art method, a canister (or bag) containing 25 grams or more or charcoal is exposed to radon bearing air. The charcoal container is designed so that the accumulation takes place over a time comparable to the 3.8 day lifetime of radon. After exposure, the charcoal container is placed in front of a gamma ray counter, usually a NaI(Tl) detector, which records the integrated gamma ray emissions. The method has several advantages: First, large amounts of charcoal can be used, since the gamma rays can penetrate many centimeters of charcoal without being attenuated. Second, because the gamma rays can penetrate out of their thin walled container, the charcoal need not be disturbed to make the measurement, and the canisters can be recycled. Third, the counting techniques are simple and well-known in the art. There are, however, serious disadvantages to the technique. First, only two of the five links in the radon decay chain are detected so that significant radon signal is ignored. Second, the gamma ray detectors used for the commercial measurement of radon have rather low efficiently; ten percent efficiently is typical. Third, the gamma ray detectors have significant backgrounds unrelated to the radon signals; increasing the detector size to improve the efficiency of detection leads to even greater increases in background counts. Fourth, humidity problems can seriously compromise the accuracy of the measurements since moisture uptake displaces the radon accumulated in the charcoal. It is difficult to maintain the dryness of the large amounts of charcoal under humid conditions, and no marketed canister has solved this problem